The photoreceptor pigment which mediates the action of light to change the phase of the circadian rhythm of conidiation in Neurospora crassa has been detected in the intact mycelium using light-induced absorbance changes as the assay. The pigment appears to be a b-type cytochrome with a flavin attached; light absorbed by the flavin causes the photoreduction of the cytochrome b. The light-induced absorbance changes persist in cell free extracts and experiments are under way to localize the pigment in specific membrane fractions. Once the pigment is localized more specific ideas as to its mode of action should become apparent. This pigment system appears to function in a large number of photoresponsive organisms. Attempts will be made to isolate the photoreceptor pigment from such organisms.